The development of high energy cell systems requires the compatibility of an electrolyte possessing desirable electrochemical properties with highly active anode materials, such as lithium, calcium, sodium and the like, and the efficient use of high energy density cathode materials, such as FeS.sub.2, Co.sub.3 O.sub.4, PbO.sub.2 and the like. The use of aqueous electrolytes is precluded in these systems since the anode materials are sufficiently active to react with water chemically. Therefore, in order to realize the high energy density obtainable through use of these highly reactive anodes and high energy density cathodes, it is necessary to use a nonaqueous electrolyte system.
Many cell or battery applications, particularly in transistorized devices, such as hearing aids, watches, calculators, and the like, require a substantially unipotential discharge source for proper operation. However, it has been found that in many nonaqueous cells employing positive active materials which include a conductive additive such as graphite and/or carbon, the cell upon initially being discharged, exhibits a high voltage whereupon the cell then proceeds to reach its lower operative discharge voltage level only after a certain time period has elapsed. The time period for the cell to reach its intended operating discharge voltage level usually depends on the discharge rate through the load and thus, depending on the apparatus it is to power, could result in a period extending up to several hours or even days. This phenomenon has serious drawbacks when a cell is intended to be used in electronic devices requiring a substantially unipotential discharge source for proper operation. In some of these electronic devices, any initial voltage peak substantially exceeding the intended operating voltage for the device could result in serious damage to the electronic components of the device. One approach to protect devices from batteries exhibiting high voltages prior to leveling off to their desired operating voltage level is to add additional electronic circuit components to protect the main operating components of the device. However, this not only adds to the expense of the device but also would result in enlarging the device to accommodate the protective circuitry. With the emphasis placed on miniaturization, it has become necessary for the battery industry to design smaller and smaller miniature power cells.
To compensate or to eliminate this initial high voltage during discharge of nonaqueous cells, it has been suggested in U.S. Pat. No. 4,018,970 that graphite and/or carbon which is normally employed as a conducting material in the cathodes of these cells be eliminated and replaced with another electronic conductor. It also discloses that when the electronic conductor has, in relation to the postulated decomposition of the electrolyte, an overvoltage such that the decomposition takes place only at a potential lower than the positive active material reduction potential, then the electrolyte decomposition does not take place before the discharge of the positive active material, since the latter reduction maintains the electrode at a potential which is too high for the decomposition to take place. Thus, the teachings in this reference basically require that the normal conductive materials of graphite and/or carbon, which are normally employed in cathodes, be eliminated.
Accordingly, it is the primary object of this invention to provide a nonaqueous cell having a cathode comprising an active cathode material, a conductive material of graphite and/or carbon, and a minor amount of a metallic reducing agent which will effectively eliminate the initial high voltage usually exhibited during the initial discharge of the cell.
Another object of the present invention is to provide a nonaqueous cell which employs a cathode having a graphite and/or carbon additive to improve the conductivity of the cathode and which exhibits a substantially unipotential voltage output during discharge.
Another object of the present invention is to provide a nonaqueous cell employing a cathode comprising an active cathode material, graphite and/or carbon, and a metallic reducing agent either within the cathode or in electrical and ionic contact with the cathode such that during initial discharge of the cell, a substantially unipotential discharge voltage will be exhibited.
Another object of the present invention is to provide a nonaqueous lithium cell employing an FeS.sub.2 cathode containing a minor amount of graphite and/or carbon along with a metallic reducing agent and which cell will exhibit a substantially unipotential discharge voltage.